I recently ran across this ebook. They show you how to get a decent amount of members on your forum in a short period of time, with no investment. They firmly state that they got 400 members in 7 days. I'm not guaranteeing that number, but it does give you some great tips and sites to help you boost popularity. The number of members you will get varies on the site you are promoting, how well you promote it using their methods, and a few other things. It should be a good read for forum owners though.